Patent literature 1 discloses a device having both a roller for a preliminary bending which has a tapered surface and a roller for a regular bending which has a cylindrical shape and is spline-fitted to an outer circumference of the roller for preliminary bending. The roller for the regular bending coaxially moves relative to the roller for the preliminary bending. The preliminary bending of a circumferential edge portion of a workpiece is performed in such a manner that the tapered surface of the roller for the preliminary bending abuts on a flange in a state where the roller for the preliminary bending protrudes more than the roller for the regular bending. Subsequently, the roller for the regular bending is moved forward and the roller for the preliminary bending is accommodated in the roller for the regular bending, and then the roller for the regular bending performs the regular bending on the circumferential edge portion of the workpiece subjected to the preliminary bending.
However, in a piece of technology disclosed in patent literature 1, the preliminary bending is performed by the tapered surface of the roller for the preliminary bending which has a small diameter allowing the roller for the preliminary bending to be accommodated in the roller for the regular bending. Thus, in some cases, the flange subjected to the preliminary bending has a wave shape. The reason for this is as follows. In the tapered surface of the roller for the preliminary bending having a small diameter, the circumferential ratio between a large-diameter portion and a small-diameter portion in the tapered surface is great and the radius of curvature on the small-diameter portion side is small. In addition, the regular bending is performed by the roller for the regular bending having a large diameter. Thus, when an obstacle is located on the flange side, for example, when an inner panel protrudes up to a portion above the circumferential edge portion of the workpiece on which hemming processing is to be performed, the roller for the regular bending is likely to interfere with the obstacle. Accordingly, in some cases, the regular bending cannot be properly performed.